1. Field of the Invention
The present invention relates to a photomask technique used for fabrication of a semiconductor integrated circuit or the like. In particular, it relates to a half-tone phase-shift mask that has a phase-shift multilayer film to attenuate light of an exposure wavelength, a mask blank used for fabricating the mask, and fabrication methods thereof.
2. Description of the Related Art
Typically, photomasks used in a wide variety of technical fields including manufacture of semiconductor integrated circuits, such as ICs, LSIs and VLSIs, are fabricated by patterning of a photomask blank, which has a transparent substrate and a light shielding film primarily made of chromium. Typically, the light shielding film is patterned by photolithography using ultraviolet rays, an electron beam or the like as an exposure beam.
In order to meet the recent market demand for higher integrating density of semiconductor integrated circuits, finer patterning has become desirable. Conventionally, the finer patterning has been achieved by shortening the exposure wavelength. However, although it can improve the pattern resolution, shortening the exposure wavelength disadvantageously reduces the depth of focus and the process stability and adversely affects the product yield.
One of pattern transfer methods that can avoid such disadvantages is a phase-shift method, which uses a phase-shift mask as a mask for transferring a fine pattern. In practice, the phase shift masks can be generally classified into fully transparent phase-shift masks and half-tone phase-shift masks according to the light transmission characteristics of the phase shifter (phase-shift film). Here, the fully transparent phase-shift mask is a mask whose phase shifter has a light transmittance equal to that of the exposed part of the substrate (that is, a mask that is transparent to light of the exposure wavelength). On the other hand, the half-tone phase-shift mask is a mask whose phase shifter has a light transmittance that is several to several tens percents of that of the exposed part of the substrate.
Among others, the half-tone phase shift mask that has the simplest structure and is the easiest to fabricate is a “single-layer half-tone phase-shift mask” that has a phase shifter composed of a single-layer film. For example, there has been proposed a single-layer half-tone phase-shift mask that has a phase shifter (phase-shift film) made of an MoSi-based material, such as MoSiO and MoSiON, (see Japanese Patent Laid-Open No. 7-140635, for example).
The mask blank (phase-shift mask blank) used for fabricating a phase shift mask is required not only to have predetermined values of optical properties including transmittance, phase difference, reflectance, index of refraction and the like at the exposure wavelength but also to be improved in durability, such as chemical resistance, and be less defective from the view point of fabrication process. However, since the phase shifter of the single-layer half-tone phase-shift mask described above is composed of a single-layer film, setting the optical properties thereof to be desired values uniquely determines the film composition. Therefore, it is difficult to provide a phase shift film that can meet practical requirements of other properties including chemical resistance.
To avoid such a problem, a layer for meeting the requirements of the optical properties and a layer for meeting the requirements of the other properties including chemical resistance can be separately prepared and stacked to form a phase-shift film composed of a multilayer film. However, there has been discovered no film structure or film composition that meets the requirements of the optical properties while achieving a satisfactory chemical resistance.
Furthermore, different film compositions results in different degrees of etching (different etch levels) in the dry etching step of the phase-shift film. Thus, there is a problem that such a difference in etch level leads to reduction of smoothness of the end face (side wall) of the patterned area, which in turn inhibits stable and precise transfer of a fine pattern.